The science of making sourdough bread

If bread is rising, sourdough is soaring. Along with pasta and toilet rolls, flour was among the first products to vanish from supermarket shelves and Covid-19 inspired a home-baking boom. While Google searches for “bread” tripled in the UK in the weeks after mid-March, those for “sourdough” rose sixfold. Sourdough differs from most bread in that it contains no baker’s yeast, relying instead on a fermented “starter” of water and flour to provide lift. This also provides its sour flavour and chewy texture.

It could be the reduced availability of baker’s yeast in shops, or perhaps the time-poor with a theoretical interest in sourdough finally found time to don their aprons. Whatever the reason, the sourdough revival has gone into overdrive. Vanessa Kimbell, author of The Sourdough School and regular contributor to Radio 4’s The Food Programme, says she has seen a 50% increase in Instagram followers and a 25% increase in membership of her online Sourdough Club, and that “the phone hasn’t stopped ringing”.


San Francisco’s Josey Baker

San Francisco’s Josey Baker. Locals insist the city’s environment gives their sourdough a unique taste, but scientists counter that the microbes involved are found globally. Photograph: DPA/Alamy

However, many sourdough newbies have found that producing beautiful, tasty loaves isn’t as easy as it looks on social media. Maybe it’s time for a new approach. The transformation of dough into a light and airy loaf is, after all, chemistry in action, with a good side of physics. The complex relationships between the yeasts and bacteria that power starter fermentation are the domain of the microbiologist. Whether you’re a beginner left holding bricks that taste of dough, or an accomplished amateur looking to further up your game, perhaps what you need is a refresher in the science of sourdough.

What should I feed my starter?

To replace the baker’s yeast that most breads need to rise, sourdough requires some starter – a self-sustaining fermentation of flour, water, wild yeasts and bacteria that produce lactic and acetic acids. You might be able to get some starter from a local baker, or you can make your own – by combining small equal quantities of water and flour and letting it ferment for around five days.

Once you have established a starter you need to feed and maintain it with additional flour and water. All-purpose, refined wheat flour can be used. Most enthusiasts prefer stoneground wholegrain flours because they contain more of the microbes and enzymes needed to drive fermentation. They can, however, make a starter higher maintenance, requiring precise timing of feeding. Many opt for a more forgiving rye flour-based starter, and some use flour blends.

“When I fed our starter sifted bread and whole wheat flours, it would ebb and flow in health, activity and consistency,” says San Francisco baker Josey Baker (yes, that’s his real name). “I’ve had the best luck with wholegrain rye flour, which seemed to stabilise it in a way that has worked very well for us for years.” An underperforming wheat flour starter that has lost its vigour can be pepped up with a rye flour boost. Some mix in extras like grapes or yoghurt, though this isn’t necessary if you’re using good flour.


Some bakers add yoghurt or grapes to their starters to boost the bacterial content of their dough; purists say this is unnecessary.

Some bakers add yoghurt or grapes to their starters to boost the bacterial content of their dough; purists say this is unnecessary. Photograph: Catherine Hoggins/Alamy

How does my starter’s diet affect bread flavour?

Flours consist mostly of starch (70-80%) and proteins (10-15%). During fermentation, enzymes in the flour break the starch apart into smaller, more digestible units, making sugars available to yeasts and bacteria. The microbes digest these sugars, generating carbon dioxide, acids and alcohol as byproducts.

Rye-based starters attract more heterofermentative lactic acid bacteria, which produce vinegary acetic acid and give bread sour, fruity flavours. Wheat flours encourage homofermentative lactic acid bacteria, which only produce lactic acid, making for softer, cereal flavours, while wholewheat adds a malty, nutty taste.

Use of more water and warm weather encourages microbes that will generate a sweeter-tasting loaf. To add sourness, rest your dough in the fridge, possibly overnight, as lower temperatures favour the activity of bacteria that produce acetic acid.

Where do the microbes in my starter come from?

Some bakers in San Francisco, which is famous for its sourdough, swear their loaves can’t be produced elsewhere. There is a species of lactic acid bacteria called Lactobacillus sanfranciscensis, which produces a characteristically sour flavour, although it has been found in sourdoughs across the world.

Scientists at North Carolina State University (NCSU) used DNA sequencing to identify the microbes in starters sent to them by more than 500 participants in their Global Sourdough Project. “We found there is incredible diversity across different sourdoughs, even those from the same neighbourhoods,” says microbiologist Dr Anne Madden, of NCSU. In a follow-up study, the team asked 18 bakers from 14 countries to make starters using the same flour and method, and then assembled them in Belgium to make bread. The microbes in their starters were most similar to those in the flour they used, but were also reflected in those found on the bakers’ hands. “This suggests an intimate relationship,” says Madden. “Either the hands are adding microbes to the starters or the starters are adding microbes to the hands.” The group hope to find out more through the Wild Sourdough Project, launched in April, in which they are asking citizen scientists to make starters and record their characteristics.


A man's hands kneading bread dough

Kneading boosts the development of gluten in the dough, the key factor in giving a loaf the good structure that will help it rise properly. Photograph: Victoria_Hunter/Alamy Stock Photo

Should I knead, or stretch and fold?

It depends whether you need your bread by lunchtime. Despite much talk to the contrary, there is no gluten in dry flour. Rather, it contains proteins called gliadin and glutenin, which, once hydrated, combine to form gluten, which can absorb twice its weight in water. It is also sticky, stretchy and elastic. The strength and structure of a good loaf rests on a scaffolding of fine strands of gluten. “Your gluten network acts like a balloon that keeps the gas produced by the yeast inside,” says Karl De Smedt, who runs a library of sourdoughs in St Vith, Belgium. The collection currently maintains 128 sourdoughs from 25 countries, some of which date back to the 19th century. “Without a good gluten network your bread cannot rise and you will end up with a brick,” adds De Smedt.

Kneading adds energy and oxygen to dough, speeding the development of the gluten, allowing speedier baking. In “no knead” recipes, edges of the dough are stretched and folded back in, and then left to develop. This process is repeated several times, before the dough is left to swell for a longer period. “You only need to knead if you want to be quick,” says Kimbell. “But if you give your dough a proper, vigorous mix, you can also let the gluten develop during a long, slow fermentation.”

How do I get a more open crumb?

Lots of factors affect the “crumb”, or internal structure, of your loaf. Many amateur bakers want to know how to produce lighter bread with larger holes. “To achieve a very open crumb, you need a good flour with a high protein, say 13-16%,” says De Smedt. “This allows you to produce a nice gluten network which is responsible for retaining the gas.”

The water content of your dough is also key. Beginners often slop in too much too quickly and then panic when their dough becomes gloopy and so throw in extra flour. Kimbell’s top tip is to start with about three-quarters of the water in your recipe, mix slowly and add splashes as you go. Greater water content makes for softer dough and bigger bubbles. “It is easier to blow up a balloon with thin skin than one with thick skin, and by adding more water your dough becomes more like a thin balloon,” says De Smedt.


Vanessa Kimbell and her Dutch oven

Vanessa Kimbell and her Dutch oven. Trapping water vapour during baking can delay crust formation; this allows the loaf to ferment and expand for longer. Photograph: Georgia de Lotz

If you want larger air pockets, be gentle when handling your dough, otherwise you’ll knock out the gas needed to make them. But while big holes make for nice Instagram pictures, some focus on structure at the expense of more important attributes. “Any fool can make big holes once they understand gluten and water,” says Kimbell. “The true art is in creating something that is complex, soft, tender, nourishing with both sweetness and light sourness.”

Why do I need steam during baking?

Sugars and amino acids on the loaf’s surface start to react at about 150C in the Maillard reaction, a process that gives seared foods a distinctive taste and colouring. Add steam into the oven and it will condense on the cold surface of your loaf and delay crust formation. Keeping the bread’s surface flexible for longer allows for a final burst of fermentation and gas bubble production, while also allowing existing bubbles to expand. It allows the loaf to stretch so that it becomes light and airy. Many cook their sourdough in a Dutch oven or under a cloche to trap water vapour from the dough and create steam.

Is sourdough healthier than baker’s yeast bread?

The lactic acids in sourdough neutralise phytates in flour that when consumed in large amounts can prevent the body absorbing minerals such as iron, calcium, magnesium and zinc in bread. They also slow the release of glucose into the bloodstream, lowering the bread’s glycemic index and preventing insulin spikes. The lactic acids also make gluten more digestible, and reduce the chances of gluten intolerance. A study published in December by Prof Marco Gobbetti, of the Free University of Bozen-Bolzano, in Italy, found sourdough to be more digestible than bread made with baker’s yeast. Sourdough fermented for longer periods made healthy individuals feel more full more quickly, and those who ate sourdough had higher amino acid concentrations in their blood.


A particularly artful loaf from Hove baker Paul Robinshaw, @_flour_water_salt on Instagram.

A particularly artful loaf from Hove baker Paul Robinshaw,
@_flour_water_salt on Instagram.

Is making sourdough a science or an art?

That depends who is making it. De Smedt believes it is best approached as a science. Josey Baker believes that to do it properly requires both perspectives. “Ultimately bread is made, not in labs but in homes and bakeries, by people with subjective opinions using their skills, intuitions, emotions and other intangibles that science doesn’t attempt to address,” he says. “If you’re trying to make bread relying only on one or the other, then you’re leaving something important out.”

Kimbell agrees that while science can point bakers in the right direction, there’s much more to making a great sourdough loaf than manipulating variables based on data analysis. “You can totally geek out on the amazing science of the microbial fermentation process,” she says. “Or you can slow down, lose yourself in the moment and fall in love with it. You use your hands, your heart and your mind to get to know the dough, feel it, understand it, and sense what it needs. The only other time you get to connect like that is, let’s be honest, when you meet a new lover.”

The Guardian

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